A 54-year-old woman had altered mental status and confusion of 2 days’ duration. She had been receiving chemotherapy for 5 months for metastatic pancreatic adenocarcinoma. Other relevant medical history included deep venous thrombosis, anemia, chronic hyponatremia, osteoporosis, irritable bowel syndrome, and seasonal allergies. She did not use tobacco, alcohol(Drug information on alcohol), or illicit substances. Cranial nerve function was intact but ocular examination revealed bilateral horizontal nystagmus. An MRI study of the brain revealed abnormal signaling in the periaqueductal gray matter.